9. The Master
Easily the best part about Series 12, Sacha Dhawan's incarnation of The Master could handily return for another series.
Throughout the modern era of Doctor Who, each Doctor has had their own Master, with only Peter Capaldi facing off against two different versions of his nemesis. It would be interesting if Davies plays with this idea and treats Dhawan's version of the character in the same way as Anthony Ainley's.
Ainley's Master fought off against the Fourth, Fifth, Sixth and Seventh Doctors with Ainley having to vary his performance depending on the Doctor that he was facing. This would be very interesting if this could carry over and we see Dhawan lock horns with the 14th Doctor.
It would be a shame if this version of The Master is wrapped up at the end of Series 13, but here's hoping we get more from Sasha Dhawan's Master in series 14.